As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and voltage, a long lifespan, and a low self-discharge rate, are commercially available and widely used.
As positive electrode active materials for such lithium secondary batteries, lithium-containing cobalt oxides such as LiCoO2 are mainly used. In addition, lithium-containing manganese oxides, such as LiMn2O4, which has a spinel crystal structure, and lithium-containing nickel oxides, such as LiNiO2, are also used. In particular, lithium-containing manganese oxides, such as LiMnO2 and LiMn2O4, are advantageous in that they contain manganese, which is an abundant and environmentally friendly raw material. In addition, it is possible to manufacture a high-capacity lithium secondary battery using such lithium-containing manganese oxides. In recent years, therefore, lithium-containing manganese oxides have attracted attention as a positive electrode active material for lithium secondary batteries. As a negative electrode active material, carbon-based materials are mainly used, and the use of lithium metals, sulfur compounds, etc. is also considered.
In the case in which the efficiency of a positive electrode and the efficiency of a negative electrode are adjusted so as to be similar to each other, it is possible to minimize inefficient usage, or waste, of the electrode. For example, in the case in which a positive electrode having an efficiency of 100% and a negative electrode having an efficiency of 100% are used, a battery has an efficiency of 100%. On the other hand, in the case in which a positive electrode having an efficiency of 90% and a negative electrode having an efficiency of 100% are used, a battery has an efficiency of 90%. That is, 10% of the negative electrode is needlessly wasted.
In particular, in the case in which a carbon-based material is used as a negative electrode active material and a high-capacity lithium-containing manganese oxide as a positive electrode active material, the irreversible efficiency of the negative electrode during initial charging and discharging, including the first charging, is 90% or higher, whereas the initial irreversible efficiency of the positive electrode is 80 to 90%.
In addition, irreversible operation of the electrode having high irreversible efficiency is caused due to the difference in irreversible efficiency between the positive electrode and the negative electrode. In order to prevent such irreversible operation, it is necessary to use a larger amount of active material of the negative electrode having high irreversible efficiency.
In order to adjust the efficiency of the positive electrode and the efficiency of the negative electrode so as to be similar to each other at the time of designing the secondary battery, therefore, an irreversible additive may be added to the positive electrode and/or the negative electrode.
In this case, however, lithium escapes from the positive electrode during initial formation, and then the irreversible additive becomes an inactive material, with the result that the energy density of the positive electrode is reduced.
On the other hand, high loading of the electrode is necessary to increase the capacity of the battery. In this case, however, the thickness of the electrode is increased, with the result that it is difficult for the electrode to be completely impregnated with an electrolytic solution. In addition, the concentration of the electrolytic solution is polarized, with the result that the resistance of the electrode is increased, whereby the output of the electrode is reduced.
Therefore, there is a high necessity for technology that is capable of solving the above problems.